The term “radio frequency filter (hereinafter, referred to as ‘filter’) refers to an apparatus for performing a function of passing or filtering signals in any predetermined frequency band, which is classified into a low pass filter, a band pass filter, a high pass filter, a band stop filter and the like according to a frequency band to be filtered.
The filter has an important characteristic such as an insertion loss and a skirt feature. The insertion loss means electric power to be lost when the signals pass through the filter, and the skirt feature means a steep extent of a passing band and a filtering band of the filter.
The insertion loss and the skirt feature have a trade-off relationship with each other according to the number of stages (degrees) of the filter. As the stages of the filter increase, the skirt feature becomes better but the insertion loss becomes worse.
In order to improve the skirt feature of the filter without the stages of the filter being increased, a method of forming a notch (reduction pole) is generally used. This is a method capable of enhancing the skirt feature of the filter, in which the notch is formed in a specific frequency band without the stages of the filter being increased, so as to enhance the skirt feature of the filter.
There is a cross coupling method as a most general method forming the notch. A technology relating to formation of the notch using the cross coupling method is disclosed in U.S. Pat. No. 6,342,825, entitled “Band-pass filter having tri-section”, issued to Hershtig, Rafi and assigned to ‘K&L Microwave Co.’ on Jan. 29, 2002.